An unstable resonator with a phase-unifying output coupler to extract a large uniphase beam of a filled-in circular pattern

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Abstract
A novel unstable resonator to extract a large uniphase beam of a filled‐in circular pattern has been investigated both theoretically and experimentally for the application to high‐power lasers. The resonator consists of a phase‐unifying partially reflecting output coupler and a concave total reflector. The extracted laser beams have been proved to be diffraction limited by the calculations for the Fresnel numbers N0=4.16, 12.5, and the magnification factors M=1.5, 2. Also, the calculations for N0=4.16 and M=1.5 have been verified by the experiment with a silent discharge‐excited 1.5 kW cw CO2 laser.